In the process of analyzing a sample by mass spectrometry (MS), an MS system first ionizes the sample to create analyte ions. The MS system then transfers the ions into a mass analyzer, and the mass analyzer resolves the ions on the basis of the ions' differing mass-to-charge (m/z) ratios. An ion detector measures the abundance of the ions at each m/z ratio detected. The MS system then processes signals outputted by the ion detector to generate mass (m/z) spectra that provide quantitative and qualitative information regarding the components of the sample (e.g., compounds, isomers, elements, etc.).
The mass analyzer operates in a controlled high-vacuum environment, for example at 10−5 to 10−9 Torr. In some MS systems, the ion source (where ionization of the sample is performed) also operates at a vacuum pressure. In other MS systems, such as when coupled to a liquid chromatography (LC) instrument (an LC-MS system), the ion source operates at or around atmospheric pressure. An MS system utilizing an atmospheric pressure ionization (API) source requires an interface between the API source and the evacuated regions of the MS system in which the mass analyzer and other devices are located. The interface needs to effectively isolate the atmospheric-pressure region where the ions are created (the API source) from the evacuated regions where the ions are processed and measured. At the same time, the interface needs to provide a way to efficiently transport the ions from the API source into the evacuated regions after the ions are created.
A capillary (i.e., a small-bore tube) is often utilized to transfer the ions from the API source into the first vacuum region of the MS system. The capillary has a small inside bore, the diameter of which may range from a fraction of a millimeter (mm) to a few millimeters. The capillary extends through the boundary between the API source and the first vacuum region, whereby the capillary's entrance is exposed to the ionization region of the API source and the capillary's exit is exposed to the first vacuum region. Ions and gas in the API source are drawn into the capillary's entrance, transported through the capillary's bore, and emitted from the capillary's exit into the first vacuum region. Ion optics guide the ions further into the MS system and ultimately to the mass analyzer. The capillary may be metal. Alternatively, the capillary tube may be glass with an electrically resistive property (coating or bulk resistance) to allow the capillary's entrance to be placed at a relatively high voltage level while the capillary's exit is maintained at a relatively low voltage level. In this case, the ions are effectively transported through the capillary's bore because the gas drag forces on the ions in the capillary greatly exceed the ion mobility (electric) forces on the ions in the presence of the internal electric field in the capillary.
Some ion transfer devices include multiple capillary bores that are fixed in position and parallel to each other. The multiple capillary bores may be located in a front section of an ion transfer device and provide multiple inlets that receive ions from the ion source, after which the multiple capillary bores transition to a single bore for the rest of the length of the ion transfer device. Alternatively, the multiple capillary bores may extend along the entire length of the ion transfer device and also provide multiple outlets from which ions are discharged into the first vacuum region of the MS system. Conventionally, the multiple capillary bores are utilized to simultaneously provide multiple, parallel paths for ions to travel through the capillary into the first vacuum stage. This has been done to increase the number of ions transported through the capillary or the amount of heat transferred into the capillary (e.g., to enhance evaporation and desolvation).
In an ideal situation, all (100% of) ions received by the ion transfer device would be transported to the MS inlet. Unfortunately, due to the small diameter of the capillary bore(s), the ions experience many collisions with the inside wall(s) of the bore(s) during the entire time the ions travel through the ion transfer device. The ion-wall collisions cause a large amount of ion losses inside the capillary. Moreover, some ions are lost at the entrance of the capillary due to electrostatic charging of the capillary. Many efforts are aimed at reducing the ion losses. One approach is to replace a glass (e.g., fused quartz) capillary with a capillary having an inside wall that is conductive with a high electrical resistance. An example of this approach is described in U.S. Pat. No. 5,736,740, the entire contents of which are incorporated herein by reference. This approach reduces the problem of the glass wall becoming charged. The approach may improve ion transport as much as 100-fold in comparison to use of a glass capillary, and also may enable faster polarity switching in applications where the MS system is switched between detecting positive ions and negative ions. Another approach is to provide an ion inlet section specially configured to reduce ion losses at the entrance of the ion transfer device, and which in some cases may be removable from rest of the ion transfer device for cleaning or replacement. This latter approach is described in U.S. Patent Application No. US 2018/0068840, the entire contents of which are incorporated herein by reference. Despite such approaches, a need remains for continued improvements in minimizing ion losses associated with ion transfer devices.
In addition, a capillary's ability to transport ions degrades over time due to chemical deposition on the inside wall and degradation of the coating on the inside wall. Consequently, users of MS systems are forced to clean or replace the capillary frequently to maintain a consistent ion signal response and stability in the MS system. Each cleaning or replacement of the capillary requires shutting down the MS system, cooling down the capillary, and in many cases bringing the MS system down to ambient pressure. The cleaning or replacement of the capillary may require the MS system to be out of operation for several days, severely limiting productivity. One approach to addressing this problem is to provide the ion transfer device with a removable inlet section. Such configuration allows the inlet section to be removed without having to break the vacuum in the MS system, as described in above-referenced U.S. Patent Application No. US 2018/0068840. Another approach is to provide a mechanical valve in the ion path that allows the capillary to be removed without compromising the vacuum, as described in U.S. Pat. No. 5,756,995, the entire contents of which are incorporated herein by reference. However, this latter approach may raise concerns about reliability and cost.
In view of the foregoing, a need remains for improved ion transfer devices.